When loading external applications like Plaid into salesforce using CDN libraries or based on some event fire from visualforce page we may need to call lightning controller method from visualforce page, So It can be accomplished using aura:method.
One more point to note down here is aura:method executes synchronously click here to see an example of handling a asynchronous server callback response example.
Source code :- https://github.com/dineshdk/sfdcpanda/tree/master/src
After clicking on Deploy to salesforce button, Login into salesforce from top right corner to proceed with deployment.
Once deployed, Open the application in dev console and click on preview to see how it works